1. Field of the Invention
The present invention relates to cast sheet materials based on a polyurethane and polymethyl methacrylate, comprising low concentrations of polyurethane, such sheet materials having a high impact strength as well as good optical properties. This invention also relates to a process for the production of such cast sheet materials.
2. Description of the Prior Art
The production of transparent cast sheets based on polymethyl methacrylate and a polyurethane is known to this art; see GB-A-1,239,701. The process described in said '701 patent entails, in a first stage, introducing into a mold a mixture comprising at least one vinyl monomer and precursors of crosslinked polyurethane which are devoid of any functional group copolymerizable with the vinyl monomer(s) (namely, at least one polyol and at least one organic compound containing at least two isocyanate functional groups) in such proportions that the ratio of the components of the polyurethane network to the sum of these components and of the vinyl monomer(s) ranges from to 5% to 95% by weight. In a second stage, the mold is subjected to temperature and pressure conditions permitting the successive formation of the polyurethane and then that of the polymethyl methacrylate in the presence of the said polyurethane.
EP-A-272,975 describes a process for manufacturing cast sheets having high impact strength, based on polymethyl methacrylate and polyurethane, and in which the ratio of the components of the polyurethane network to the sum of these components and of methyl methacrylate ranges from 5% to 35% by weight.
While the above prior art indicates that the cast sheets may contain a polyurethane content higher than 5% by weight, none exemplifies a sheet material having a low polyurethane content, i.e., less than 10% by weight.
The mechanical properties of cast sheets based on polymethacrylate and polyurethane which are described in the prior art vary as a function of the polyurethane content. In particular, when the polyurethane content increases, the impact strength increases. In contrast, the flexural modulus and the heat resistance (evaluated using the Vicat temperature) decrease.
Cast sheets having a low polyurethane content are of interest because of the fact that they are more stable to aging.